Effect of pre-chamber volume and hydrogen energy ratio on the performance of direct injection ammonia-hydrogen engine

Ammonia-hydrogen engines with carbon-free emission characteristics are a promising solution to realize the transition to carbon neutrality for heavy-duty long-distance transportation. Ammonia direct injection has the potential to reduce heat transfer losses, and combustion efficiency can be further improved by integrating pre-chamber jet ignition technique, but research is still limited. This study conducted a numerical investigation of a direct injection ammonia-hydrogen engine to analyze the effects of pre-chamber volume and hydrogen energy ratio on performance. Results indicate that excessively large or small pre-chamber volumes lead to increased heat transfer losses and unburned losses, respectively, thereby reducing thermal efficiency. Furthermore, large-volume schemes require a more mass of hydrogen to maintain optimal pre-chamber hydrogen concentration, whereas small-volume schemes demand a higher hydrogen energy ratio to improve main chamber combustion efficiency. A moderate-volume pre-chamber balances heat transfer and unburned losses, achieving higher thermal efficiency and reducing dependence on hydrogen energy. Through spark timing optimization for a 1.5 % volume ratio pre-chamber with a 3.5 % hydrogen energy ratio, a maximum indicated thermal efficiency of 53.02 % was attained. Compared to baseline spark timing, NH3 and N2O emissions decreased by 14.5 % and 21.6 %, respectively.